With the continuous development of power systems, the application of vacuum circuit breakers is also increasing, which is inseparable from their advantages. However, there are always some drawbacks. For example, the overvoltage problem of vacuum circuit breakers has affected their development speed to some extent. Therefore, we need to find ways to mitigate or prevent overvoltage in vacuum circuit breakers. Below, Huadian Vacuum Circuit Breaker | VS1 Vacuum Circuit Breaker Manufacturer will explain the types of overvoltage and protection measures related to vacuum circuit breakers.
I. Main Overvoltage Types of Vacuum Circuit Breakers
Operating overvoltages of vacuum circuit breakers mainly occur during the opening (breaking) process, and the following three types are common:
1. Current-Cutting Overvoltage
Mechanism: Vacuum circuit breakers have extremely strong arc-extinguishing capabilities. When interrupting a small AC current (such as the starting current of an unloaded transformer or motor), the current may be forcibly cut off (i.e., current-cutting) before its natural zero-crossing point. At this time, the magnetic energy stored in the inductive load (such as motor windings) will be converted into electric field energy, charging the equivalent capacitance of the load, thereby generating extremely high overvoltages.
Characteristics: The overvoltage amplitude is related to the magnitude of the current-cutting value and the load inductance. The larger the current-cutting value, the higher the overvoltage. This is the most common type of overvoltage in vacuum circuit breakers.
2.Multiple Re-ignition Overvoltage (High-Frequency Multiple Re-ignition Overvoltage)
Mechanism: After a circuit breaker trips, the distance between the contacts is very small. If the recovery voltage (power frequency recovery voltage) between the contacts is higher than the insulation strength of the contact medium during interruption, the gap will break down again (re-ignition). Due to the extremely rapid recovery of the vacuum medium, this process may occur multiple times within microseconds (high-frequency re-ignition). Each re-ignition is accompanied by the extinction of a high-frequency current and the reinjection of energy, continuously pumping energy into the load through a cumulative effect, causing the overvoltage to rise step by step.
Characteristics: This type of overvoltage has a high steepness and large amplitude, posing a significant threat to the inter-turn insulation and longitudinal insulation of motors. It mainly occurs when interrupting high-voltage motors or unloaded transformers.
3. Capacitive Load Interruption Overvoltage
Mechanism: When interrupting a capacitor bank, high-amplitude and high-frequency overvoltages are generated due to arc re-ignition. If the circuit breaker re-ignites after interruption, it is equivalent to one group of already charged capacitors discharging to another group of capacitors through the arc, forming high-frequency oscillations, leading to a sharp increase in voltage.
Characteristics: This situation can generate extremely high overvoltages (usually 2-3 times or even higher than the voltage to ground), which seriously threatens the safety of the capacitors and circuit breakers themselves.
II. Overvoltage Protection Measures for Vacuum Circuit Breakers
For the above-mentioned overvoltage types, protection is typically achieved through a combination of "blocking" (reducing the current-cutting value) and "draining" (providing a discharge path or absorbing energy). Common protection measures include the following:
1. Installing a Resistor-Capacitor Absorber (RC Absorption Device)
Principle: Consists of a resistor (R) and a capacitor (C) connected in series and in parallel on the load side.
Capacitor (C): Reduces the load's surge impedance, slows the rise of overvoltage, and thus reduces the high-frequency component of reignition overvoltage.
Resistor (R): Dissipates energy, suppresses oscillation, and prevents resonance between the capacitor and the load inductance.
Applicable Scenarios: Primarily used to protect motors and transformers, especially effective in suppressing multiple reignition overvoltages and current-cutting overvoltages.
Advantages: Stable protection effect, limiting both the amplitude and steepness of overvoltages.
Disadvantages: Larger size, higher cost, and energy consumption due to long-term resistor heating.
2. Installation of Zinc Oxide Surge Arresters (MOA)
Principle: Utilizing the excellent nonlinear volt-ampere characteristics of zinc oxide varistors. Under normal operating voltage, the arrester is in a high-resistance state (microampere-level leakage current); when an overvoltage occurs and exceeds the operating threshold, it quickly conducts, discharging the overvoltage energy to ground, thereby clamping the voltage within the range that the equipment insulation can withstand.
Type Differentiation:
Ordinary Distribution Type Surge Arresters: Primarily used to limit lightning overvoltages.
Dedicated Rotating Machine Type Surge Arresters: Specifically designed to protect motors, featuring lower residual voltage and better matching the motor's insulation level.
Applicable Scenarios: Primarily used to limit current-cutting overvoltages. It should be noted that ordinary surge arresters have a slow response speed to multiple reignition overvoltages at steep wave leading edges, and the protection effect may be limited; they usually need to be used in conjunction with RC (Resistant Current Arresters).
3. Install an inductor-capacitor (L-C) filter
Principle: An inductor (L) is connected in series with a capacitor (C) in parallel on the load side. The inductor blocks high-frequency current, and the capacitor reduces the rate of voltage change (du/dt). The combination effectively filters out high-frequency oscillation components, fundamentally slowing down the overvoltage buildup process.
Applicable Scenarios: Locations with extremely high overvoltage protection requirements (e.g., special motors, critical equipment).
4. Select a vacuum circuit breaker with a low current-cutting value
Principle: Addressing the overvoltage at its source. The current-cutting value of a vacuum circuit breaker is closely related to the contact material. Using low-current-cutting-value contact materials (e.g., CuCr (copper-chromium) material with special processing) can reduce the current-cutting value to a very low level (e.g., 0.5A - 2A), thereby significantly reducing the amplitude of the overvoltage at its source.
Evaluation: This is the most ideal protective measure, but due to material processing and cost limitations, it sometimes needs to be used in conjunction with other protective devices.
5. Install a parallel resistor or capacitor
Principle: Connect a resistor or capacitor in parallel across the circuit breaker contacts.
Parallel resistor: Reduces the rate of recovery voltage rise during reignition, decreasing the probability of reignition.
Parallel capacitor: Reduces the steepness of the recovery voltage between contacts.
Evaluation: This method is typically integrated into the circuit breaker's internal design; field installation is rare.
Summary and Selection Recommendations
In practical engineering applications, a single measure is often not used in isolation. Instead, a combination of protection measures is employed based on the value and importance of the protected object:
For ordinary transformers/lines: Low-current-cutting-value vacuum circuit breakers are typically selected, with general-purpose zinc oxide surge arresters installed on the busbar side for backup protection.
For high-voltage motors: Motors have lower insulation levels and are sensitive to steep surges. A combined protection scheme using RC absorbers (to suppress reignition overvoltages and reduce steepness) and dedicated surge arresters for rotating electrical machines (to limit energy-type overvoltages) is recommended.
For capacitor banks: It is recommended to install gapless zinc oxide surge arresters (dedicated to capacitor protection) on the circuit breaker side, preferably close to the capacitors. Sometimes, series reactors are also needed to suppress inrush currents and overvoltages.
Shaanxi Huadian Electric Co., Ltd., established in 2007, has been deeply involved in the field of power distribution switch control equipment for fifteen years. We not only participated in the R&D of the mainstream domestic permanent magnet vacuum circuit breaker VS1/VEGM, but also invest nearly 7%-8% of our annual revenue in R&D, solely to overcome industry pain points and safeguard the safe operation of power systems. Our main VEGM series adopts a solid-sealed pole design. Utilizing APG technology, the vacuum interrupter and upper and lower outgoing terminals are completely embedded in high-performance epoxy resin. This not only completely solves the problem of dust and moisture erosion of the insulation, but also enhances the circuit breaker's resistance to mechanical shock. Whether in the extreme cold of high altitudes or the dusty environment of industrial plants, Shaanxi Huadian circuit breakers can operate stably. Please contact us:pannie@hdswitchgear.com.
